/*
 *
 *	This uses the same logic as crc_16 but generates the crc required for
 *	the kermit protocol. This is the standard CCITT crc equation that  is
 *	described  in  September  1986  BYTE  note to get SDLC crc start with
 *	0XFFFF in stead of 0x0000 - Rick
 *
 */


#include <stdio.h>
#define	EXT	extern
#include "crc.h"

static unsigned			_crc;					/* CRC accumulator			*/
static unsigned char	checksum;				/* Checksum accumulator		*/

/* Something to force next table to be word aligned	*/
static char		unused;

static unsigned
crctable[256] = {   /* CCITT crc */
	0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
	0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
	0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
	0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
	0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
	0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
	0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
	0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
	0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
	0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
	0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
	0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
	0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
	0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
	0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
	0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
	0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
	0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
	0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
	0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
	0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
	0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
	0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
	0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
	0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
	0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
	0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
	0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
	0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
	0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
	0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
	0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
	};

#ifdef __WATCOMC__
/* ********** c source ********** */
unsigned
compute_crc_ccitt( n, buf )
	int		n;
	char	*buf; 
	{
	register unsigned char *s;
	register unsigned val = 0;

	if ( use_crc ) {
		for ( s = buf; n; --n )
			val = crctable[ ( *s++ ^ val ) & 0xff ] ^ ( val >> 8 );
		return( _crc = val );
		}
	for ( s = buf; n; --n )	val += *s++;
	return( checksum = val );
	}
#else

unsigned
compute_crc_ccitt( n, buf )
int		n;		 /* 14[bp] */
char	*buf;    /* 16[bp] */ 
{
	asm("		mov cx,14[bp]	");
	asm("		mov bx,16[bp]	");

	asm("	b	test <use_crc>,#0ffH");
	asm("		jz chk0			");

	/* compute crc */
	asm("crc0:	mov si,#0		");
	asm("		mov di,16[bp]	");

	asm("crc1:	mov bx,di		");		/* *s++ */
	asm("	w	inc di			");

	asm("	b	mov al,[bx]		");		/*  crctable[ (foo ^ val ) & 0xff ] */
	asm(" 	w	xor ax,si		");
	asm(" 	w	and ax,#0ffH	");
	asm(" 	w	shl ax			");
	asm("		mov bx,#<crctable>	");
	asm("	w	add bx,ax		");

	asm("	    xchg cx,dx		");		/* (val >> 8) */ 
	asm("	    mov cx,#8		");
	asm("	w   shr si,cl		");
	asm("	    xchg dx,cx		");

	asm("	w   xor si,[bx]		");    /* val = foo ^ foo */

	asm("		loop crc1		");

	asm("	    mov ax,si		");
	asm("	    mov <_crc>,ax	");
	asm("		jmp done		");

	/* compute checksum */
	asm("chk0:	mov ax,#0		");

	asm("chk1:                  ");
	asm("	b	add ax,[bx]		");
	asm("		inc bx	 		");
	asm("		loop chk1		");

	asm("		mov <checksum>,ax");

	asm("done:					");
}
#endif

void
clear_crc_ccitt() {
	_crc = checksum = 0;
	}

unsigned
next_crc_ccitt( ch )				/* Compute the character into the crc	*/
	unsigned	ch;
	{
	_crc = crctable[ ( ch ^ _crc ) & 0xff ] ^ ( _crc >> 8 );
	checksum += ch;
	return( ch );
	}

unsigned
get_crc_ccitt() {
	return( use_crc ? _crc : checksum );
	}
